Hilary and Haylie Duff Joke Their Kids Will Go to Coachella Together in 2030

Hilary and Haylie Duff are not only siblings but also best friends — and now, it seems, their next generation is following suit.

The sisters talked the relationship between their two kids — Hilary’s son Luca Cruz, 5½, whom she shares with ex-husband Mike Comrie, and Haylie’s 2½-year-old daughter Ryan Ava Erhard, whom she shares with fiancé Matt Rosenberg — at the Words With Friends 2 launch party in West Hollywood, California, on Thursday.

“They really play well together,” Haylie, 32, told PEOPLE. “One time I had Luca for a sleepover and the two of them were outside running around on a Slip ‘n Slide, and I remember looking out at them in the yard being like, ‘This is what they’re going to do as teenagers.’ ”

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Adds Hilary, 30, “Haylie sent me a picture and Ryan had no shirt on, a diaper and a flower crown. I don’t know what [Luca] had on, but they looked like they had been playing so hard. She was like, ‘Yo, Coachella 2030.’ I’m like, ‘No! Stop!’ If this is a flash into our future, we should be very afraid.”

Though it’ll be years before Luca and Ryan are old enough to join festival-goers, they’ve recently reached some age-appropriate milestones of their own. For Ava, it has been “transitioning to big-girl toothpaste.”

“It has been a real fight in our house because she really likes the baby toothpaste,” Haylie explains. “We had a couple of matches about that, but now we brush our teeth with big-girl toothpaste.”

A big success for Hilary and Luca has been getting him to sleep the whole night in his own room.

“He would always come into my room in the middle of the night and be scared, which is so sweet,” Hilary says. “I love those cuddles more than anything, but now he stays the whole night in his room and he’s really proud of himself.”

Since Luca started kindergarten earlier this year, Hilary says she has started to notice big changes in his behavior.

“In two months, he’s a different child,” she explains. “He has always been a great kid, but the things he’s interested in now changed so fast.”

And the things he’s not interested in have changed, too. “He won’t eat pizza!” the Youngerstar says with a laugh. “Ryan wanted Luca’s pizza last night. She was like, ‘Why don’t you like that? You’re crazy.’ ”

When it comes to the tradition of making their grandmother’s stuffing recipe on Thanksgiving, though, Haylie means business.

“I think, for me, that’s the only thing I care about at Thanksgiving,” Haylie says, adding the key to what makes it special: “There’s a lot of chicken broth in it and the bread is dried out for two days. So it’s really kind of stale, but then it gets moist from the chicken broth.”

“It’s a long process,” adds Hilary. “But really good.”

When the two sisters can’t be together in person like they will be for the holidays, they keep a constant game of Words With Friends 2 going to keep in touch.

“I’m always in New York filming, and we’re always on different schedules,” Hilary says. “A lot of times we’re texting through the app. I’m such a gamer, and she always beats me.”